With high-tech innovations in performance fabrics and design, activewear is working harder than ever to keep its coolin the gym and on high-fashion runways

OLIVIA NEWTON-JOHN'S REFRAIN of "Let's get physical," along with the ubiquity of Lycra, started it in the 1980s. Aerobics helped push it into the '90s, and yoga certainly added an aura to the category in the new millennium. Chanel, Prada, Dior and Hugo Boss all capitalized on it a few seasons ago. Activewear has come a long way from the gym and, thanks to relaxed dress codes inaugurated by casual Fridays, it has become a uniform for more and more Americans. But it's not about cotton T shirts and sweats anymore. Today activewear can mean everything from a high performance, temperature-regulating...